Trailer charge system

You can not run two regulators on one alternator unless you have a high power regulator that deals with the main output. Plus it would not work well as because if the alternator output fell for some reason, the second regulator would not be able to jack it up

I WOULD SIMPLY run a continuous solenoid. THEY WORK. If you are worried about the connection with no trailer, you could add a series switch to the coil so the key cannot activate the solenoid.